OFDM air interfaces will become increasingly important e.g. for future evolutions of air interfaces in 3GPP Radio Access Networks, for Wireless Local Area Networks (WLANs) e.g. according to the IEEE 802.11a standard or for a 4th generation air interface.
In OFDM transmission, frequency patterns are allocated to the mobile terminals. Up to now, different cells have different carrier frequencies or time-frequency patterns that are random like, so that no interference coordination between the cells is necessary or possible.
Given the licensed bandwidth, transmission capacity from network providers e.g. for WEB surfing or video streaming has to be as high as possible for all users to serve as many subscribers as possible. Further the quality of service experienced by the user and the coverage of the service is an important property demanded by the user. So OFDM transmission shall also work at the cell border.
A frequency re-use factor of 1 for the different cells and interference coordination shall be achieved for OFDM transmission in order to increase the utilization of the bandwidth without degradation of the quality of service caused by inter-cell interference.
In cellular systems with a frequency re-use factor of 1 the signal to interference ratio at the cell border approaches the factor 1 or 0 dB, so that no useful transmission from the base station to the mobile terminal can be kept up. Therefore in CDMA systems (CDMA=Code Division Multiple Access) soft handover was introduced using a different code from the neighboring cell in addition to the primary code from the serving cell. For packet transmission using High Speed Downlink Packet Access (HSDPA) no such solution is given reducing the coverage of HSDPA transmission to a fraction of the cell area.
In OFDM transmission, frequency patterns are allocated to a mobile terminal instead of codes in CDMA systems. In OFDM transmission, in contrast to CDMA transmission, interference can be planned and avoided. For OFDM transmission, which does not provide different scrambling codes for the different base stations, the problem at the cell border has to be solved as well. For that purpose frequency patterns are allocated to the users and the caused cross-cell interference can be coordinated.
For unsynchronized base stations, frequency patterns are searched that have frequency diversity to cope with the frequency selective channel transfer function, and the pilot subgrids of the OFDM time-frequency grid are used for channel estimation.
For the coordination of interference, the frequency patterns have to be the same in neighbor cells, while the pilot subgrid in neighbor cells shall be different to allow channel estimation also in the interference region. So, these frequency patterns have to be compatible with all possible pilot subgrids, meaning that the number of pilot hits i.e. stolen subcarrier frequencies by pilots needs to be independent of the pilot subgrid. Constructing then compatible patterns of low rate becomes a problem under these conditions. Distributing the subcarriers over the frequency axis and achieving compatibility with the pilot subgrid becomes difficult if the pilot distance is no prime number which it is usually not.